The present invention relates to a drive system for at least a water pump driven by a reversible electric motor with the interposition of a rotatable sealing arrangement.
It is common practice, for example in dishwashing machines, to use at least one water pump driven by a reversible motor, preferably an asynchronous motor, whose drive shaft sealingly extends across the casing of the pump. In dishwashing machines, in particular, the water seal is usually formed by a ceramic ring which is, fixed on a partition wall between the pump and the motor, and a graphite ring which cooperates with the ceramic ring and is rotated by the drive shaft. More particularly, the graphite ring is mounted about the drive shaft on the side of the partition wall communicating with the pump, and is connected to the shaft through resilient biasing means which usually include a pressure spring and a shaped rubber ring, or the like. The pressure spring surrounds the rubber ring, that in turn is keyed about the drive shaft, and keeps the graphite ring axially pressed against the ceramic ring, and to which the rotational movement of the drive shaft is discharged due to the friction exerted by an embossed edge of the rubber ring which is interposed between the spring and the graphite ring. Basically, the rotatable water-tight seal is formed between the slidingly cooperating surfaces of the ceramic ring and the graphite ring, which to this aim are made with a high degree of finishing.
However, it is known that the two ceramic and graphite rings may easily "stick" together (that is, a high starting friction may occur between the mutually cooperating surfaces), sometimes to such an extent as to prevent rotation of the asynchronous motor, which can be damaged by overheating.
Such sticking may easily occur, mainly in a dishwasher, due to the following reasons:
deposit of dirt and/or limestone particles on the sealing rings; PA1 scaling of the sealing rings, caused by salt used for regeneration of the water softener; PA1 drying-up of the sealing rings after prolonged inoperative periods of the machine and; PA1 excessive axial pressure exerted by the spring.
Arrangements are known, as disclosed for instance in DE-B-1061425, to increase the starting torque of an electric motor. However, these arrangements relate to the use of a synchronous motor to rotationally drive a reduced load. More particularly, in order to compensate for the low starting torque of the synchronous motor, the driven shaft is coupled with the driving shaft by means of a transmission system which enables the driving shaft to idle by an angle which is smaller than 360.degree..
These prior art arrangements are, of course, substantially ineffective. For example when, as in the above-mentioned case, they are used in a dishwashing machine, wherein a remarkable starting torque may be required to enable the pump to start up correctly when sticking occurs in the rotatable sealing arrangement.